Stretch roll forming

ABSTRACT

A method of forming includes applying a stretching force along a linear axis of a work piece and forming the work piece while the stretching force is applied.

This application claims the benefit of U.S. Provisional Application,Ser. No. 61/171,247, filed Apr. 21, 2009 and entitled Stretch RollForming.

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for formingmetal.

BACKGROUND OF THE INVENTION

Complex curved metal components are formed to a desired shape by themechanical action of dies (roll forming) or by application of otherexternal forces, such as stretching, hydraulic fluid, induction coils,etc. In most roll forming processes, unique sets of dies need to befabricated and used for each specific forming operation of each part.Moreover, many shapes require at least one additional shaping process inorder to achieve the desired shape, such as stretch forming. Thisincreases the cost and lead time of production. The U.S. patent toGerald Hackstock, U.S. Pat. No. 6,286,352, entitled Stretch Roll FormingApparatus Using Frusto-Conical Rolls is exemplary of the effort tocombine the roll and stretch forming processes into a single process.

It is therefore the primary objective of the present invention toprovide apparatus and a method for improved material forming utilizingroll dies and stretching techniques simultaneously.

SUMMARY OF THE INVENTION

Prior to introducing the general and specific teachings of the presentinvention, some general observations about the physics of metal formingwill assist in the understanding of the present invention.

While in mechanics traction is typically used to refer to both thenormal and tangential forces exerted over a surface element, since theobject of the invention is to provide for methods of applyingsubstantial forces tangential to the surface, for the purpose ofclarity, in this application the usage “traction” or “traction force”refers exclusively to the component of forces applied on a surface thatare substantially tangential to the surface F_(t). The maximum value ofthe traction force that can be exerted over a surface element typicallyis the product of the normal force applied F_(n) and the coefficient offriction between the means of applying the normal and traction forcesand the surface with which said means is in contact to apply the normaland traction forces. “Normal stress” σ_(n) refers to the normal forceper unit area of the surface. “Traction stress” refers to the tractionforce per unit area of the surface. If the traction force were uniformlyapplied over the surface element, the local traction stress at any pointwithin the surface element would be the same as the average tractionstress. The traction stress at the surface is a shear stress that inmany cases decreases at points away from the surface, deeper inside thebody. If the body is in static equilibrium under the action of theforces imposed on it, this gradient of shear stress causes a tangentialstress σ_(x) within the body in a direction perpendicular to the surfacenormal.

If opposing normal forces are applied over opposite faces of a body, thebody would be in equilibrium due to the normal forces canceling eachother. However, the body would be subjected to compressive stress equalto σ_(n). The maximum normal force that can be applied is limited by theability of the body to withstand the compressive stress withoutyielding. The magnitude of the compressive stress sustainable may belimited by other stresses in the body. For instance, if there is atangential tensile stress σ_(x) along the direction parallel to thesurface (say this is along the length direction of a flat extrusion),equal to 50% of the yield strength (Y) of the material of the body andif the normal stress applied at the contact equals 50% of the yieldstrength, the body would begin to yield. If there is no stress in thethird direction (along the surface, perpendicular to the tensile stress,then the plastic strain in the body would be tensile along the tensiondirection (ε_(⊥)x>0) and compressive along the normal direction[(ε]_(n)<0). If at the contact, a traction stress were applied inaddition to the normal stress (say the traction stress is equal to ½ thenormal stress (τ=σ_(⊥)n/2), then the body would begin to yield evenbefore the normal stress equals 50% of the yield strength. UsingTresca's criterion, the normal stress would be about 29% of the yieldstress when yielding begins. However, if the normal and tractionstresses were applied over a width which is at least equal to thethickness of the body, the bulk of the body would begin to yield whenthe normal stress reaches about 25% of the yield stress.

Taking into account the effect of the traction on increasing thetangential stress (tensile stress parallel to the surface is equal tothe increased value on the right side of the contact) it can be seenthat yielding will actually begin at the top right corner of the contacteven before the normal stress reaches 25% (actually at 23.6%) of theyield stress. Thus it is likely that there may always be a little morestrain at the contact surfaces than subsurface and will lead to theburnishing effect identified earlier. The magnitude of this can becontrolled by spreading the normal and traction forces over differentdistances, that is, changing the stresses σ_(n) and τ.

The above description and conclusions are true at all grips acting toapply a tensile stress within a body. For instance, at the grips whichgrip a tension test specimen in a tensile tester, or at the grips whichgrip a sheet in a drape forming press, or at the grips which grip anextrusion in an extrusion stretch press, if the specimen gripped had auniform cross section (like in the last two examples above) and thegrips were to attempt to stretch the specimen to yielding, it will beseen that the end of the contact between the grip and the specimen iswhere the effective stress is greatest, causing the material to yieldfirst there.

Grasping a specimen via rollers, so that the specimen moves with respectto the grips, causes the stretch as well as the burnishing strains to beuniformly spread throughout the specimen, permitting much more strain,hardening and compressive residual stress on the surface, therebyleading to an overall lighter and stronger part.

Normal and traction stresses may be applied to a body on only onesurface and not on the opposite surface. Depending on the configurationof the body, this may lead to other effects such as a bending moment.

The present invention is based on the concept that sets of rollers canbe mounted on suitable mechanisms and used as reconfigurable ‘dies’ thatguide the formation of parts into desired shapes. The same rollers, whensuitable driving torque is applied to them, are also used to stretch thepart while it is being formed. The stretch-roll aspect of the formingoperation also includes bending of the metal work piece while a tensileforce is exerted on it. In operation, the work piece may be a sheet or astructural section such as a T-section or pipe. The work piece is formedusing a plurality of roller sets that exert a tensile force on the workpiece to form it into a desired shape. The tensile loading that isexerted while forming the part reduces spring back and residual stressesin the fabricated component. The stretch of the work piece is controlledby the rotational speed of the roller sets and the torque on therollers. The position and orientation of the roller sets is dynamicallyconfigured in one or more planes to control and vary the contour alongthe length of the work piece. Sensors provide feedback to adaptivelymeasure and control the stretch in each section of the work piece aswell as its geometry.

The advantage of using many sets of relatively small rollers to stretchcomponents, as opposed to using two big rolls on opposite sides to gripa part and pull it in opposite directions, as shown in the Hackstockpatent, U.S. Pat. No. 6,286,352, is that each contact between a rollerand the component can be small. Thus, normal and stretch forces can betransmitted into curved parts of a work piece without flattening themout. The small stretch forces exerted by each roller set addcumulatively, leading to stretch forces large enough to stretch partsplastically.

The process of using multiple sets of rollers to grasp and stretchcurved parts without flattening them can be employed with tractorelements which apply normal and stretch forces over larger contiguousareas of the sheet.

At each point of contact, the curvature of the tractor element matchesthe desired curvature of the part. This is accomplished using diesegments for parts of constant curvature. For parts with variablecurvature along the stretch direction, this can be accomplished using“flexible raceways”. Additionally, for parts with a gentle curvature,tractor elements of a different curvature, or no curvature, clamp overfinite lengths of the part to establish the stretch in regions away fromthe section where the stretch is greatest and where the bending isexpected to be done. The maximum permissible length of a tractor elementdepends on the difference in curvature, thickness of the rubber tractorbelt used and the maximum bending moment that the section can withstandwhile still being elastic, which depends on the stretch level thatexists at that particular section. For example, tractor elements of 4 ftradius may be able to exert most of the stretch force, even if theradius of curvature of the part is 4.5 feet. The advantage of tractorelements is that much larger stretch forces can be generated whilegrasping the part over a limited length. In the following specificationand claims, when rollers or roll sets are referenced it, it should beinferred to include tractor elements.

DESCRIPTION OF THE DRAWINGS

In the drawings, which are not necessarily drawn to scale, like numeralsmay describe similar components in different views. Like numerals havingdifferent letter suffixes may represent different instances of similarcomponents. The drawings illustrate generally, by way of example, butnot by way of limitation, various embodiments discussed in the presentdocument.

FIG. 1 illustrates consecutive sets of rollers.

FIGS. 2A, 2B, and 2C illustrate formed work pieces.

FIGS. 3A, 3B, and 3C illustrate fabrication of a formed work piece.

FIGS. 4A, 4B, and 4C illustrate views of rollers and a ‘T-channel’ workpiece.

FIG. 4D illustrates a set of rollers having a convex profile.

FIGS. 5A and 5B illustrate roller stations.

FIGS. 6A and 6B illustrate roller stations.

FIGS. 7A and 7B illustrate work pieces and selected rollers.

FIGS. 8A and 8B illustrate plate stacks.

FIG. 9 illustrates a track roller configuration.

FIG. 10 illustrates a roller station.

FIGS. 11A and 11B illustrate rollers.

FIGS. 12A, 12B, and 12C illustrate linear element rollers.

FIG. 13 illustrates an arrangement of linear element rollers.

DETAILED DESCRIPTION

Roll forming is a fabrication process of forming a work piece to adesired geometry by applying suitable forces by a plurality of rollerstations that are precisely located and shaped with respect to a workpiece. Forming is accomplished by relative motion between the work pieceand a plurality of roller stations. The work piece can be sheet stock ora segment of stock of some preformed shape. In the stretch roll formingof the present invention, specific torque is applied to one or more ofthe rollers to cause a stretch that assists in forming the part into thedesired shape while at the same time reducing residual stress in thework piece.

FIG. 1 illustrates the basic system 10A of the present invention. Eachof a plurality of sequentially arranged roller stations 30A, 30B, 30C,and 30D includes a set of rollers, each set having at least two rollerswhere a roller is sometimes referred to as a die.

In the embodiment illustrated in FIG. 1, a flat strip is stretched andformed to a radius. Each roller station includes two rollers on oppositesides of work piece 20A, for instance, roller station 30A includesrollers 40-A1 and 40-A2. Work piece 20A is formed while traveling in thedirection of arrow 5. The force applied by the rollers in rollerstations 30A and 30B are in a manner to oppose the flow of work piece20A. The forces applied by roller stations 30C and 30D are controlled tooppose the force exerted by roller stations 30A and 30B and to provide asmall additional amount of forming force. More specifically, rollerstations 30A and 30B have clockwise torque applied to the upper rollersand counterclockwise torque applied to the lower rollers, resulting in atensile force acting to pull the work piece 20A to the left, whileroller stations 30C and 30D have opposite torque applied to theirrollers that exerts a tensile force pulling the work piece 20A to theright, akin to a tug-of-war. In the illustrated embodiment, rollerstation 30D is driven in a speed controlled mode and pulls the workpiece 20A through from left to right.

Work piece 20A is propelled through system 10A by at least one poweredroller of the system 10A. For example, roller 40-A1 can be powered androller 40-A2 can be unpowered. More than one roller of system 10A may bepowered. For example, roller station 30D may provide speed control tomove the work piece through system 10A.

Work piece 20A can be a sheet of metal, a plate, an extrusion, a wire, atube or other work piece. It may initially be in a flat form or coiledand after forming can be bent or straightened. In FIG. 1 the work piece20A is initially straight and a radius is formed therein by passing itthrough system 10A. The radius has a curvature governed by the spacingand location of the individual roller stations. The curvature of thepathway through system 10A imparts a corresponding bend in the workpiece 20A which is incrementally formed over a series of stations whereeach roller station imparts additional deflection or forming. In eachpass through the system, the work piece is shaped between the two rollerstations where the stretch is at a maximum and later roller stations(those to the right of FIG. 1) serve to unload work piece 20A withoutimparting additional forming. System 10A may be configured to form aparticular curvature or to straighten a work piece. Three or more setsof roller stations are used to form a curve in a work piece and two ormore roller stations are used to straighten a work piece.

The system 10A may remain stationary while the work piece travels in thedirection shown by arrow 5. On the other hand, the system 10A may travelin a direction opposite that of arrow 5 while the work piece 20A remainsstationary. A “pass” includes relative movement of the work piece andthe system 10A.

Consider an example in which the tensile strength of the work piece hasyield strength of 10,000 PSI and an ultimate strength of 15,000 PSI. Inthis example, a 5,000 pound tensile stress (stretch force per unitcross-section area) will allow forming the work piece with reducedresidual stress and spring back. In general, increasing the tensilestress exerted while roll forming produces a formed work piece havingreduced residual stress and reduced spring back. The amount of tensilestress may take on a value (above 10,000 PSI for the above example) suchthat the stretch may be in the range of 2% to 3% although other valuesare also contemplated.

The force applied by a particular roller can be controlled by aprocessor or other controller. In the case of a roller powered by a DCmotor, the force can be controlled by modulating the current supply. Aroller can be powered by an electric motor, a hydraulic motor, or othersource of rotary power delivered directly or through a variety of powertransmission means such as a gear, a chain, a belt, a shaft or othermeans.

In FIG. 1 sensors 50A and 50B provide an electrical output signal basedon the measured strain in the work piece. At least one of the sensorsmay be an optical sensor to monitor the stretch between adjacent rollerstations. Sensors 50A and 50B may include encoders that measure therotation of selected rollers and a processor coupled to the sensors todetermine the strain in the work piece using the difference in rotaryspeed. Each roller station may be followed by a sensor to monitor thestretch and to determine uniformity in a process control environment.Alternatively, both optical sensors 50A and 50B may directly measure thevelocity of the sheet and encoders measure the rotation of the rolls inorder to monitor the slip of the rollers with respect to the work pieceand accordingly control the normal force exerted by the rollers on workpiece.

In FIG. 1 the vertical positions of roller station 30A, 30B, 30C, and30D are individually adjustable in order to provide the desired formingoperation. Each roller station is threadingly adjustable on a verticalaxis.

System 10A includes adjustable components to allow various formingoperations including imparting a stretching force and performing rollforming. System 10A may be a component of a CNC (computer numericcontrol) machine.

Material with other cross-sections may be similarly stretch bent orstretch roll formed, as illustrates in other Figures of the attacheddrawings. The rollers at each roller station may include additionalrollers that are positioned around the work piece and at differentpositions along work piece 20A. The rollers may be arranged in pairs.

FIGS. 2A, 2B, and 2C illustrate stretch roll formed sheet material. Inthe examples illustrates, a stretching force is applied predominantly inthe direction indicated by the central arrow. The work piece istransformed by the applied force, the geometry, location, and shape ofthe rollers comprising each of the roller stations. Stretch force can beapplied in a direction parallel to or perpendicular to the plane of thecontoured shape. The contour can be in the plane of the stretch force(i.e. along a plane perpendicular to the axis of the rollers) asillustrates in FIG. 1, or it can be in a plane perpendicular to thestretch force. The stretch force may be applied by a plurality ofrelatively short length rollers, each roller having an axis tangentialto the local contour. The work piece can be formed to have a doublecurved surface with a first contour along the direction of the stretchand a second contour perpendicular to the stretch. In addition, theclamping (or normal force) exerted by the rollers can be varied alongthe direction of the axis of the rollers (along the length of thecontour) to result in a curvature in the plane of the sheet. Forexample, a particular roller can have a contoured profile that mayinclude a taper, a crown or dish-shaped configuration.

In FIG. 3A, work piece 20A is formed by rollers 54. Rollers 54 can bepart of a particular roller station or can be independentlycontrollable. FIG. 3B illustrates the normal forces applied to a workpiece 20B by rollers 54. Arrow 52A denotes a large normal force, arrow52B denotes a medium normal force and arrow 52C denotes a low normalforce. The resulting curvature in work piece 20A is illustrates in FIG.3C.

FIGS. 4A, 4B, and 4C illustrate views of rollers and a ‘T-channel’ workpiece. FIG. 4A illustrates a view of roller stations A-A and B-B, havingrollers 410, 412A, and 420, among others. Roller station A-A includesroller 410 as well as roller 412A and roller 412B (as shown in FIG. 4B).Rollers 410, 412A, and 412B exert a normal force on the horizontalsegment of the work piece 20C. FIG. 4B also illustrates frame element310 and frame element 320 coupled by threaded shafts 330. Threadedshafts 330 include left-handed thread portion and right-hand threadportion and engage corresponding internal threads of frame elements 310and 320 to control the compressive normal force exerted on work piece20C. A plurality of roller stations corresponding to the exampleillustrates in FIG. 4B can be arranged sequentially and used to formcurvature around the pitch axis shown in FIG. 4A.

In FIG. 4B, rollers 412A and 412B have a common axis in parallel withthe axis roller 410.

Roller station B-B is illustrated in FIG. 4C and includes rollers 420Aand 420B. A threaded shaft 430 engages internally threaded carriers toadjust the spacing of rollers 420A and 420B in order to exert acompressive force on the vertical web of work piece 20C. The threadedshaft 430 includes a left-hand thread portion and a right-hand threadportion. Roller station B-B can be used to form curvature around the yawaxis shown in FIG. 4A. Roller stations A-A and B-B can be used incooperation to form curvature in the roll axis shown in FIG. 4A. Inaddition, complex curvature can be formed by combinations of rollerstations A-A and B-B.

Roller stations A-A and B-B can be part of a forming station similar tothat shown in FIG. 1. The rollers can be configured to have acylindrical shape or other solid shape. For example, a roller caninclude an axi-symmetic shape such as the frustum of a cone.

As illustrated in FIG. 4A, a work piece can be formed to have a bend orcurvature denoted by roll, pitch, and yaw. A particular bend can beformed by exerting a stretching force and a rolling force using a seriesof rollers (such as those of roller station A-A and roller station B-B)configured with a particular location and orientation. Rollers 420A and420B can be configured to engage the vertical leg (web) with sufficientcompressive force to maintain substantially uniform alignment of the legwhile avoiding buckling, kinking, or collapsing of the leg or toincrease the stretch exerted by the cumulative effect of rollers orother roller stations.

Rollers of individual roller stations can be repositioned. Repositioningpermits changing the location and the orientation of axes. In addition,different roller stations can be repositioned in order to control orfollow the movement of the work piece as it progresses through theforming system.

The rollers exert a compressive force in order to achieve a slightthinning of the cross section of the work piece. The compressive forceinduces a compressive residual stress at the surface of the work piece.The rollers may include rubber coated metal wheels or elastic wheels.Each roller is interchangeable or selectable in order to accommodate avariety of different cross-sections of the work piece.

A work piece can be formed by changing the location of the rollerstations as the work piece moves with respect to the roller stations. Inthis configuration, the pitch, roll, and yaw can change smoothly.

FIG. 4D illustrates rollers 441 and 442 having a convex profile. Rollers441 and 442 rotate on parallel axes. The convex profile may be suitablefor forming a work piece having a circular cross section. The work piececan be tubular (such as pipe) or solid round section (such as wire orrod). Other roller configurations are also contemplated includingcomplex face profiles tailored for forming a particular work piece. Theexamples illustrated include face profiles that match the particularwork piece as well as other profiles that can be used to produce desiredchanges in the geometry of the cross-sectional shape.

As shown in FIG. 5A, rollers 410, 412A, and 412B are coupled to a frame510. The frame transmits equal and opposite roll forces (the normalforces) at each roller station. The rollers can be built around, orinside of, frame 510. For instance, the frames for the sets of rollers410, 412A, and 412B at each roller station can be combined into onerigid body. In this configuration, the frame 510 takes up the equal andopposite roll forces (the normal forces) exerted by each pair of rollersand a mechanism can be configured to position the frame (station) at aparticular location and orientation. The mechanism supplies the nettensile force at each particular station. Such a mechanism includesadjustable length links coupled between neighboring stations and havingjoints at the connections to the frame at each station. Thisconfiguration carries the compressive forces to react to the tensileforce through the work piece.

Frame 510, shown in FIG. 5A, is configured to form a work piece in amanner corresponding to the roller station of FIG. 4B. Adjustable links512 and 514 are coupled to roller 410 and can be configured toindependently, or jointly, adjust alignment and position of the axis ofroller 410. In a similar manner, adjustable links 516 and 518 arecoupled to rollers 412A and 412B, respectively, and can be configured toindependently, or jointly, adjust alignment and position of the axis ofroller 412A and the alignment and position of the axis of roller 412B.

Frame 520, shown in FIG. 5B, is configured to form a work piece in amanner corresponding to the roller station of FIG. 4C. Shaft 430 iscoupled to carriers 435 which carry rollers 420A and 420B. Rollers 420Aand 420B can be moved together or apart by rotation of shaft 430. Frame520 carries the load exerted by the rollers 420A and 420B.

FIG. 6A illustrates frame 610 coupled to frame 620. Frame 610 and frame620 can include a roller station as described elsewhere in thisdocument. A work piece can be passed through the frames and formed bystretch rolling. In FIG. 6A, the frames are shown to be held in a fixedalignment.

In FIG. 6B, frames 630, 640, and 650 are in adjustable alignment.Adjacent frames are coupled by a system of adjustable links. Frame 640is coupled to frame 650 by links 660A, 660B, 660C, and 660D. A greateror smaller number of links can be used and in one example, the ends ofthe links are terminated with an articulating joint, such as a sphericalor a universal joint.

Links 660A, 660B, 660C, and 660D can include a threaded shaft, ahydraulic shaft, a pneumatic actuator or other type of adjustable linearelement. The links are operated to adjust the alignment and relativeposition as to adjacent frames, and thereby control the contour formedby passage of the work piece.

An orienting mechanism may be employed to control the orientation of theframe at each roller station. For example, an articulating mechanism,such as a robot or a similar structure, provides the reaction force tosupport the traction force exerted by the rollers on the work piece.

In one embodiment including an articulating mechanism, a link is coupledbetween adjacent frames to take up the traction force and to reduce theloading on the articulating mechanism. The link can include sphericaljoints or universal joints at one end or at both ends. In thisconfiguration a robot provides the moment to react to the load exertedby the offset between the axes of the work piece and the link joiningthe neighboring frames.

The relative orientation of adjacent frames is controllable by a systemof links coupling the adjacent frames. The system of links can include alink with a spherical joint along with at least one other link having anadjustable length. For instance, adjacent frames are coupled by a linkhaving a spherical or universal joint and two or more links having anadjustable length. The length of a link can be adjusted by a hydrauliccylinder or by a threaded screw mechanism. The links between adjacentframes can be oriented and spaced around the work piece to control therelative origination of the neighboring stations and to sustain theforces and moments exerted by the work piece.

In addition to the opposing roller configurations shown in some of theFigures, other configurations, including non-opposing rollers, are alsocontemplated. For example, one roller configuration can form a workpiece having a variety of cross sections. Some examples of crosssections include angle stock, I-beam, and hat channel. The cumulativeeffect of the rollers at each of the roller stations are used to gripthe work piece and to stretch the work piece, and the relative locationand orientation of the roller stations can be controlled to produce apart having a specified geometry.

The set of rollers within the frame of a roller station can becontrolled. For example, the location of at least one of a part ofopposing rollers can be selected to control the roll force or the normalor clamping force exerted by the pair of rollers.

A mechanism such as a hydraulic cylinder or a screw can be providedwithin a frame so that the location of the rollers can be controlled andchanged in order to form the work piece. The work piece can be graduallychanged in the size or shape of its cross-section either from one run tothe next, or within the run for each work piece. The work piece can bepassed through a progressive series of roller stations or the rollerstations can be moved over the work piece.

The configuration of a roller can be selected to achieve a particularresult. As shown in FIG. 7A, the length of the roller can be selected toenable formation of a curved panel. The roller length is increased andthe roller profile is configured to correspond with the finishedcontour. An increased roller length can result in a reduction in thenumber of motors at each station. However, the spacing between rollersof a particular frame at a particular roller station having rollersdistributed around the perimeter of the cross-section, would increaseand the curvature of the work piece would only be approximately enforcedby the rollers.

In the example shown in FIG. 7B, the diameter of the rollers isincreased to reduce the number of roller stations for a given formingoperation. However, an increased rolled diameter is accompanied by anincreased spacing between adjacent stations, that is, in the lengthdirection of the work piece. Furthermore, an increased roller diametermay lead to stress non-uniformity throughout the work piece and to anincreased minimum length for the work price.

Other configurations for the present subject matter are alsocontemplated.

The individual or discrete rollers in a roller station are separatelypowered. This configuration may not be suitable for certainapplications. For example, the acquisition and maintenance costs forindividually powered rollers, such as a combination of a motor and aroller, may be burdensome. In addition, the roll forces exerted on awork piece may not be uniformly distributed along the cross section ofthe work piece or along the length of the work piece.

A track element may be configured to contact the work piece over arelatively large area rather than small area of contact provided byindividual rollers. The track element can be support in a curvedconfiguration using the reinforcing structure illustrated in FIGS. 8Aand 8B. In FIG. 8A, stack 810 is generally planar and is retained inposition by the clamping force exerted by a pair of threaded fasteners.In FIG. 8B stack 820 is also held in a curved configuration by a pair ofthreaded fasteners. The track element 910 resembles those in a trackedvehicle, as shown in FIG. 9. The track elements serve to increase thearea across which the load or force is exerted and may provide a moreuniform distribution of loading. Track element 910 is configured toapply a normal force and a tensile traction to the surfaces of asheet-type or an extrusion-type work piece in a manner that allowsincreased uniformity of the normal and traction forces.

Consider an example in which the curvature of the cross section of thework piece is small or negligible so that only the contour in the planeof the bend is important. In this case, the contour in the plane of thebend (for example, the length direction of the work piece) iscontrolled. The rollers 912 are configured as needle rollers, that is, ahigh aspect ratio in which the roller length is much longer that theroller diameter, which press on the inner surface of an endless belt orthe track element 910. The endless belt 910 is driven by two largerollers 915. The normal force that presses the needle rollers 912 intothe belt is provided by a flexible raceway or stack 810 located on theinterior of the belt. The flexible raceway stack 810 includes alaminated stack of well-lubricated, smooth sheets such as shown in FIGS.8A and 8B. The sheets are bent easily by applying a suitable force ormoment and then locked in position by means for arresting the relativesliding motion of the sheets. The force for pressing the raceway ontothe rollers may be provided by a bladder placed inside the belt or itcould be provided by small hydraulic cylinders 925 such as those used ina fixturing system. The rollers 912 are coupled together by a drivechain 930. The drive chain is routed around a circumference within theperimeter of the belt 910 so that the rollers do not have to travelaround the pulleys driving the belt.

Since low sliding speeds may suffice under most conditions, the rollers912 may be eliminated under well lubricated conditions and in that casethe normal force is exerted on the raceways and thereby against therubber belt 910. In another embodiment, the rollers are attached to theplates and the rollers rotate about their own axis. This configurationmay be suitable for an application utilizing larger diameter rollerswhere the length of the rollers is of the order of their diameter. Therollers may have a cambered profile to improve the uniformity of thenormal stress applied. Rather than using a roller, a rotatable ball maybe held in position. In that case the roller or the ball is coupled withcompliance of the raceways to allow bending in the plane including theaxis of the rollers. This configuration may improve the uniformity ofnormal stress exerted on the work piece. The uniformity of stressexerted by the belt can be improved by increasing the thickness of thebelt and by having segmented pieces of softer rubber facing on the steelbelted rubber tread portion that is driven by the rollers. Where thecurvature of the cross-section of the work piece is small, a pluralityof tractor elements can be configured to act on different chords of thecross-section in order to affect the traction for stretch bending.

FIG. 10 illustrates a configuration of rollers in which the position andalignment of individual rollers is independently selectable. Work piece20C is formed using rollers 1010, each of which is positioned by a firstand second links 1020. Links 1020 are coupled to a frame 1030 and arelinearly adjustable to achieve a desired force and forming pressure.

FIG. 11A illustrates apparatus configured for forming a work piece inwhich the curvature of the cross-section of the work piece is high andchanging and the curvature in the length direction of the work pieceremains small. For instance, a leading edge skin for a wing formed frominitially flat sheet stock will have high curvature in one direction andlittle curvature in a perpendicular direction. In this apparatus, rigidrollers 1110 are spaced around the circumference at each station andadjacent rollers are interconnected using flexible coupling 1115 such asa universal joint configured to transmit torque from one roller to thenext roller. The flexible coupling arrangement reduces the number ofnecessary drive motors. The shape of the set of rollers is controlled byindividually controlling the location of the rollers or by controllingthe shape of a flexible raceway which exerts a force on the rollers. Therollers may have individual rubber liners and in one embodiment a singlebelt envelopes the rollers in the length direction. Such a belt may alsoenvelop rollers at more than one cross-section, if the curvature fromsection to section changes gradually. In FIG. 11A, sheath 1120 includesa belt or an elastic sleeve that provides a greater area of contact withthe surface of the work piece.

FIG. 11B illustrates apparatus in which rollers 1110 are coupled by aflexible coupling 1115 and further supported by adjustable links 1130.Work piece 20D is formed by stretch roll forming via a normal forceexerted by the adjustable links 1130.

FIGS. 12A, 12B, and 12C illustrate various configurations for linearelements suitable for use with the method of the present invention. Alinear element, such as those referenced at 1210, 1220, and 1230, isfixed at a proximal end and includes a bearing surface, such as 1215,1225, and 1235, disposed at a distal end. Bearing surface 1215 includesa captivated ball. Bearing surface 1225 includes a roller on an axisaligned substantially normal to the main axis of the linear element1220. The bearing surface can be offset from the main axis, similar to acastor wheel, to improve alignment and ease passage of a belt, asillustrated by bearing surface 1235.

FIG. 13 illustrates a number of linear elements 1210A, 1210B, 1210C, and1210D held in fixed alignment. The spacing between adjacent linearelements is controlled by interconnecting links comprising hydrauliccylinders 1250A, 1250B, 1250C, and 1250D. The height of each bearingsurface 1215 and the track element or belt is positioned in the desiredconfiguration. The links 1250A, 1250B, 1250C, and 1250D between adjacentlinear elements supply a force holding the linear elements together orapart at the desired distances.

The described track elements are suitable for accurately forming a workpiece in which the local curvature at each cross-section is relativelyhigh. The track elements may also allow forming of a work piece having awidely varying curvature from one cross-section to another along thelength of the work piece.

In addition to stretching and forming, other fabrication procedures canalso be performed. For example, and with reference again to FIG. 1, afabrication procedure can be performed before passing work piece 20Athrough system 10A, between selected roller stations of system 10A, orafter passing work piece 20A through the system.

What is claimed is:
 1. A method of forming comprising, applying astretching force in a region where a work piece is being formed, along alength of the work piece by controlling a torque applied to a pluralityof sets of rollers that contact the work piece, wherein controlling thetorque includes operating a first set of rollers with a firstpredetermined torque to pull the work piece in a first direction andoperating a second set of rollers with a second predetermined torque topull the work piece in a second, opposite direction; and, simultaneouslyroll forming the work piece in the region where the stretching force isbeing applied, while stretching an entire cross-section of the workpiece along its length, wherein roll forming includes bending the workpiece to have a radius along at least one of a yaw axis, a pitch axis,and a roll axis.
 2. The method of claim 1 wherein applying thestretching force includes, exerting a tensile force by the use of aplurality of sets of rollers in contact with the work piece.
 3. Themethod of claim 1, wherein the radius is determined by the relativelocations of one or more rollers of the sets of rollers.
 4. Then methodof claim 1, wherein the radius is not the same as that of a roller ofthe sets of rollers.
 5. A method of applying traction forces to asurface of a work piece through at least one grip comprising, contactingthe work piece with the at least one grip over two or more contactregions along which the surfaces of the work piece and the at least onegrip are substantially conformal and where the at least one grip and thework piece exert normal forces on each other, where at least twotraction forces are transmitted to the work piece, wherein contactbetween the at least one grip and the work piece occurs at two or morerollers, balls, or tractor elements which are pressed onto the surfaceof the work piece and are driven so as to apply the at least twotraction forces to the work piece, where the at least two tractionforces act in opposing directions and stretch an entire cross-section ofthe work piece over a stretching region; and simultaneously plasticallydeforming the stretching region to have a radius along at least one of ayaw axis, a pitch axis, and a roll axis.
 6. A work piece forming systemcomprising: a plurality of consecutive set of rollers for exerting astretching force along a linear axis of a work piece where each set ofrollers comprise at least one roller disposed proximate to each of theopposed top and bottom surfaces of a work piece, and a controllercoupled to the sets of rollers to control the torque applied to at leastone set of rollers.
 7. The system of claim 6 and further including, aframe to which the plurality of consecutive sets of rollers are affixed,the frame having adjustment means for controlling the position of atleast one set of rollers.
 8. The system of claim 6 and furtherincluding, at least one sensor configured to measure strain existing inthe work piece between sets of rollers.
 9. The system of claim 6 wherethe controller receives an input signal from at least one sensor. 10.The system of claim 6 and further including, first endless loop tractorbelts each having inside and outside surfaces and interconnectingadjacent rollers proximate to the top and bottom surface of the workpiece where the outside surface of each belt makes contact with arespective surface of the work piece, and a flexible raceway comprisinga plurality of stacked plates disposed proximate the inside surface ofeach tractor belt.
 11. The system of claim 10 and further including, aplurality of needle rollers positioned between the respective racewaysand the inside surface of the respective endless loop belt.
 12. Thesystem of claim 10 and further including, a second endless and rotatableloop belt into which the needle rollers are serially attached, saidsecond endless loop belt disposed within the loop of the first endlessloop belt.
 13. The system of claim 12 and further including, a pluralityof hydraulic cylinders in operative contact with the raceways forselectivity providing bending forces to the stacked plates.
 14. A methodof applying traction forces to a top and bottom surface of a work piececomprising: providing first sets of rollers disposed proximate to eachof the top and bottom surfaces of the work piece, where each of thefirst sets of rollers comprises at least one roller, enveloping thefirst sets of rollers proximate to the top surface of the work piece bya first endless loop tractor belt; enveloping the first sets of rollersproximate to the bottom surface of the work piece by a second endlessloop tractor belt; said first and second tractor belts each havinginside and outside surfaces; providing one or more flexible racewaysurfaces disposed proximate the inside surface of each tractor belt;wherein the flexible raceway surface acts to press a portion of theoutside surface of the respective tractor belt against the respectivesurface of the work piece, wherein each of the one or more flexibleraceway surfaces are bendable between configurations of differentcurvature; and controlling a torque applied to at least one roller ofeach of the first sets of rollers to drive the endless belts.